Yes, that was exactly what I was looking for! Can't believe I missed that one, went trough collect expand etc. Thanks a lot!

Yes, that was exactly what I was looking for! Can't believe I missed that one, went trough collect expand etc. Thanks a lot!

Thanks for your comment! In the equation where I want to use this, both variables also appear seperately, and I only want to subsitute in the case where they appear together (as a multiplication). Two seperate substitutions would substitute all the occurences I guess.

For completeness, I'm trying to do a Galerkin decomposition of the Euler-Bernoulli beam equations. The complete equation is:

The first part should be subsituted by the relation 

giving:

Where E, Ii, M, T and A are constants.